It is well known to tone electrostatic latent images contained on photoconductive or dielectric surfaces by application thereto of so called monocomponent or single component electroscopic marking particles or toner which may be of the heat fixing or cold pressure fixing type.
Single component toner particles contain magnetisable material such as for instance magnetite whereby such toner can be applied for toning purposes to the latent image bearing surface by means of a magnetic applicator roll. It is customary to have the single component toner contained within a hopper or tank from which it is fed onto the applicator roll. The applicator roll consists usually of a non magnetic stationary or rotating sleeve within which is contained a rotating multi-pole magnet roll. As such magnet roll rotates, the magnetic field lines associated with the poles usually of alternating polarity attract toner to the non-magnetic outer sleeve and cause said toner to form a coating thereon, commonly referred to as the magnetic brush. A so called doctor blade is usually fitted to control the coating thickness or the height of the magnetic brush. The magnetic brush is brought into close proximity to the latent image bearing surface to effect toner transfer over the so called toning gap by attraction to the latent image areas where the electrostatic forces associated therewith overcome the magnetic forces holding the toner against the sleeve of the applicator roll.
The machine performance that is image quality and stability of single component toners depends on a number of properties of the toner material such as electrical conductivity, tribocharging characteristics, particle size and particle size distribution, etc., as is well known in the art. One of the main factors affecting machine performance of the toner is related to its flow properties as is also well known. By good flow properties is meant the ability of the toner to distribute itself uniformly and rapidly over the surface of the outer sleeve of the magnetic applicator roll so that imagewise removed toner material by the latent image bearing surface is instantly replenished and thus the magnetic brush in the toning gap is of constant height and uniformity. Poor flow properties of a toner are indicated by uneven image density mainly in the form of streaks or bands along the copy, which are caused by uneven toner distribution on the applicator roll or insufficient toner in localised portions thereof.
In order to improve the flow properties of single component toners it is customary to admix so-called flow improvers such as silica powders and the like therewith in certain specific proportions.
A common test of toner flow characteristics is measurement of the angle of repose of the toner. However such measurement can give widely varying results in repeat testing of one and the same sample due to variation in degree of aeration, amongst other things, and the test results thus obtained can not be directly related to actual machine performance.